French patent 2 728 809 discloses a pump comprising a pushbutton displaceably mounted on a support which is secured to the receptacle containing the substance to be dispensed, the pushbutton having a circularly cylindrical central duct provided with radial openings at its bottom end, the support defining an annular pump chamber of variable volume around said duct. A diaphragm made of elastomer is mounted on the support. The diaphragm has a circularly symmetrical central portion in the form of a sleeve that is open at its top end and closed at its bottom end. The central duct of the pushbutton is inserted in the diaphragm until it bears against the end wall of the sleeve. The diaphragm constitutes a resilient return member enabling the pushbutton to be returned to its initial position after a quantity of substance has been dispensed.
During the return movement of the pushbutton, the diaphragm presses against the central duct and isolates the pump chamber from the radial openings of the central duct so at to prevent air returning into the pump chamber.
Such a pump presents the advantage of having only a small number of parts and thus of being of relatively low cost to manufacture.
Nevertheless, that known pump does not provide complete satisfaction, and the Applicant company has observed, in particular, difficulties in priming.